1. Field of the Invention
The present invention generally relates to a centrifugal pellet dryer of the type which utilizes a bladed lift rotor conveying moisture laden plastic pellets or other solid particles upwardly within a cylindrical screen. The centrifugal force imparted to the particles by rotation of the lift rotor causes the particles to engage the interior surface of the screen, and moisture on the particles is discharged through the screen in a manner well known in the art. More specifically, the present invention relates to a product flow modifying deflector associated with the internal surface of the cylindrical screen.
2. Description of the Related Art
Centrifugal pellet dryers are well known in the art for separating water or moisture from plastic pellets and other solid particles, such as a slurry of water and plastic pellets produced by underwater pelletizers. Centrifugal pellet dryers of the prior art include a vertically disposed outer housing, a cylindrical screen oriented in the housing and a driven bladed rotor positioned centrally inside the screen. The rotor moves water laden pellets or other solid particles upwardly within the screen with centrifugal forces imparted to the particles by radial air flow from the rotor (see FIG. 1) causing the particles to move radially outwardly into engagement with the screen for discharge of water through the screen. The dried particles are discharged from the upper end of the screen and housing, and water is discharged from the lower end of the housing.
Centrifugal pellet dryers of this type are disclosed in U.S. Pat. Nos. 7,171,762, 7,024,794, 6,807,748, and 6,237,244, commonly owned by the assignee of this application. In the operation of such dryers, the pellets or other particles being moved vertically and radially by the bladed rotor engage the cylindrical screen with substantial velocity and usually bounce off the screen back toward the rotor for imparting further vertical and centrifugal forces to the particles as they are moved upwardly inside the screen. This is depicted by the “good” flow characteristic illustrated in FIG. 2. As further shown in FIG. 3, the “best” flow of both product and air occurs when the radial air flow from the rotor does not just push the pellets but actually flows around them.
However, conventional centrifugal dryers used in the market today all have a common problem relating to the air flow created by the normal rotation of the rotor. The combination of rotor blade geometry and other physical factors creates an air flow that can greatly affect the flow of the product through the dryer as it bounces between the rotor and the screen.
Furthermore, with the advent of newer plastic materials which form softer pellets, or pellets with flat or lentoid geometries, and the making of very small pellets, or so-called micropellets, using underwater pelletizers, difficulties have been encountered in conveying and subsequently drying such pellets in known centrifugal dryers. In addition, known centrifugal dryers have encountered difficulty in conveying and subsequently drying ground flake plastic materials which are formed from recycled soda bottles, milk containers and the like, as well as certain other plastic particles such as ground battery casings.
More specifically, and as depicted by the “poor” flow characteristics in FIG. 2, softer and smaller pellets, pellets with flat or lentoid geometries, and plastic flakes, as well as certain other plastic and similar particles, tend to collect and circulate in the clearance band “X” (see FIG. 2) between the outer edges of the rotor blades and the inner surface of the screen. Rather than bouncing around in the manner of harder and larger pellets or particles, these particles become trapped against the screen by the air flow. This undesirable circular flow and resultant entrapment of the softer and smaller pellets, pellets with flat or lentoid geometries, and plastic flakes and particles along the inner surface of the screen is sometimes referred to as “banding”. This banding reduces product flow through the rotor area of the dryer and increases power requirements for maintaining rotational speed of the rotor. Further, it has been found that banding also reduces the efficiency of moisture separation from the solid particles, can cause high amperage requirements within the dryer, and reduces overall efficiency of the centrifugal dryer. These problems often result in fines and fiber-like “hair” production (often referred to as angel hair in the industry).
The problem of banding is particularly evident with pellets having a flat or lentoid geometry as the relatively large planar surface area of this shape most naturally causes the pellets to adhere to the inner surface of the screen and, because of the associated low profile of such pellets, makes them difficult to dislodge. As illustrated by the “worst” flow in FIG. 3, the larger the product's surface area in one dimension, or the more flake-like or lentoid the product, the greater the opportunity for the outward air flow of the rotor to trap the product against the screen. This phenomenon greatly reduces the necessary bounce required to reengage the product with the outward and upward action of the dryer rotor.
One solution for overcoming this problem of banding is set forth in U.S. Pat. No. 6,739,457 (“the '457 patent”), which is commonly owned by the assignee of this invention. The disclosure of the '457 patent is hereby expressly incorporated herein by reference as if fully set forth in its entirety.
In the '457 patent, deflector strips are fastened to the inside of the dryer screen using multiple fasteners fitted within countersunk holes machined within the strips. This method of fastening results in the deflector strips being relatively expensive to manufacture and also necessitates that the screen also be provided with dedicated holes which can create undesirable stress concentrations within the screen. In addition, should the fasteners become loosened, either through vibration, aging or other cause, there is the risk that the deflector strips could extend into the moving rotor with resulting damage. Further, any spacing between the deflector strip and the screen may collect portions of the pellets or other foreign matter, particularly with pellets having a flat or lentoid geometry, thus leading to possible contamination in future product runs.